Conventionally known is a fixing device for heating a sheet using a plate-shaped heat generating member. This fixing device is configured such that the surfaces of the plate-shaped heat generating member and a pressure roller face each other. This fixing device is configured such that the plate-shaped heat generating member is in contact with the inner surface of an endless belt and the opposite surface of the endless belt is in contact with a first surface of a sheet, thereby heating the sheet via the endless belt. This fixing device is also configured such that the pressure roller and the second surface of the sheet are in contact with each other, allowing the plate-shaped heat generating member and the pressure roller to produce pressure. This allows the fixing device to fix a toner image transferred to the sheet onto the sheet.
The endless belt is in contact with the pressure roller. When the pressure roller has a high heat capacity, the heat for heating the endless belt is taken away by the pressure roller, and at warm-up or when returning from sleep, this will cause a delay corresponding thereto in reaching a specified temperature. In this context, for example, it is conceivable that during temperature raising such as at the time of warm-up, the pressure roller is separated from the endless belt to eliminate the path through which heat escapes to the pressure roller, thereby improving the performance of temperature raising of the fixing device.
However, in this case, the contact region of the endless belt with the heat generating member may be excessively heated, thus possibly accelerating the speed of deterioration of the endless belt.